Cholecystokinin(CCK) plays a key role in facilitating digestion within the small intestine. It is secreted from mucosal epithelial cells in the first segment of the small intestine (duodenum), and stimulates delivery into the small intestine of digestive enzymes from the pancreas and bile from the gallbladder. Cholecystokinin is also produced by neurons in the enteric nervous system, and is widely and abundantly distributed in the brain. Cholecystokinin and gastrin are highly similar peptides. Like gastrin, cholecystokinin is a linear peptide that is synthesized as a preprohormone, then proteolytically cleaved to generate a family of peptides having the same carboxy ends. Full biologic activity is retained in CCK-8 (8 amino acids), but peptides of 33, 38 and 59 amino acids are also produced. In all of these CCK peptides, the tyrosine seven residues from the end is sulfated, which is necessary for activity.